Many polymers have been developed which can withstand exposure to irradiation, often incorporating specific additives or antioxidants which prevent polymer degradation. Films and multilayer films made from these and other polymers are sometimes exposed to irradiation during processing or use. For example, irradiation is used in some instances to destroy micro-organisms in the wrapped product (e.g., in food sterilization) and to also prepare at least one component of the film for thermal shrinkage at a later time.
Multilayer film packaging has many requirements for both enduring the irradiation exposure and preparing part of the structure for subsequent shrinkage. Typically, the package is a multilayer structure, with the different layers performing different functions. Usually the layers are made from thermoplastic polymers. For example, one layer may be a barrier layer (e.g., ethylene vinyl alcohol (EVOH)), the heat seal layer may be a polyethylene (e.g., linear low density polyethylene (LLDPE)), while still another type of polyethylene (e.g., high density polyethylene (HDPE)) might be used as the structural or core layer.
Irradiation of multilayer film structures can be done by exposure to Cobalt 60 (gamma irradiation) or by electron beam (beta). When the entire multilayer film structure is irradiated, the heat sealability of the heat seal layer often decreases (i.e., the intiation temperature of the heat sealing increases), especially when the heat seal layer is polyethylene, while the physical properties (e.g., strength) of the structural layer increase. Reduction in heat sealability causes lower heat seal strengths and narrower heat sealing temperature ranges. Exposure to irradiation, especially beta irradiation, and subsequent exposure to heat can also cause the core layer and the heat seal layer to shrink, especially when the multilayer structure has been oriented (e.g., film). Controlled shrinkage of the core layer is desirable, but a narrower heat sealing temperature range of the heat seal layer is not desirable.
Irradiation of the heat seal layer also results in higher heat seal initiation temperatures as well as narrower heat seal temperature ranges. The narrow heat seal range of the heat seal layer can cause the wrapped product to become exposed to the external environment and contamination, thus negating the sterilizing effects of irradiation. A better balance of heat seal and shrinkage properties of the overall structure is needed before and after irradiation so that the overall integrity of the package is not lost after irradiation.